1. Field of the Invention
This invention relates to semiconductor processing and, more particularly, to a method for in-situ cleaning of a quartz furnace used for the deposition of poly-crystalline silicon upon a wafer topography.
2. Description of Relevant Art
Fabrication of a field-effect transistor ("FET") is well-known. The manufacturing process begins by lightly doping a single crystal silicon substrate. The active areas where the transistors or other devices will be formed are then isolated from other active areas on the substrate using isolation structures formed in the field regions. A gate dielectric is formed in the active regions, preferably by thermally oxidizing the silicon substrate. Subsequently, a gate conductor is patterned upon the gate dielectric. Source and drain regions are laterally displaced on either side of the gate conductor. A channel region between the source and the drain is protected from the implant species by the pre-existing gate structure. When voltage above a certain threshold is applied to the gate of an enhancement-mode transistor, the channel between the source and drain becomes conductive and the transistor turns on.
The original FETs used metal to form the conductive gate structures which gave rise to the name metal-oxide-semiconductor ("MOS"). However, poly-crystalline silicon ("polysilicon") has taken the place of metal as the preferred gate conductor material. Metal processing tends to be "dirty" resulting in contamination of the substrate with unstable threshold voltages. Furthermore, metals typically have a relatively low melting point which restricts the processing temperatures subsequent to metal deposition. Polysilicon has a higher melting point and can permit higher temperatures to be used subsequent to deposition.
Polysilicon is typically blanket-deposited upon the wafer and then patterned to form the gate conductive structure using conventional photolithography. Chemical vapor deposition ("CVD") is the preferred method of depositing polysilicon. Vapor phase chemicals that contain the required constituents react together to form a solid film. CVD can be performed in special reactors which are held at atmospheric pressure. For the deposition of polysilicon, low-pressure CVD ("LPCVD") reactors are preferred over the earlier atmospheric pressure CVD reactors. LPCVD reactors offer better step coverage, less particulate contamination, and excellent uniformity.
An example of a vertical flow LPCVD reactor is shown in FIG. 1. Quartz 10 is the preferred material used to form the inner lining of the furnace chamber due to its purity and high temperature rating. Heaters 12, which are between housing 13 and quartz 10, help maintain the furnace chamber ambient at a particular temperature. Typically, 500-700.degree. C. is the preferred temperature used for the deposition of polysilicon. Wafers 14 are placed inside a carrier, often referred to as a "boat". Boat 16 is then placed vertically inside the furnace chamber. Vapor phase chemicals enter the furnace chamber through input port 18, which thereafter directs the chemicals toward wafers 14 using injectors 20. Silane or dichlorosilane are typically injected into the furnace chamber. Upon entering the furnace chamber, the gases decompose to produce polysilicon. Reaction byproducts exit the furnace chamber through output manifold 22. Manifold 22 is connected to a vacuum pump which maintains a low pressure, for example, 0.25-2.0 torr inside the chamber.
Polysilicon is not only deposited upon wafers 14 but, unfortunately, also upon quartz inner lining 10. After substantial use of the furnace, a thick film of polysilicon layer 24 accumulates on the quartz inner lining. Polysilicon layer 24 becomes an increasing source for contaminants which can cause defects upon wafers 14. Therefore, after approximately 100 hours of operation, quartz inner lining 10 needs to be replaced. The process requires disassembling/reassembling the furnace which is time consuming and results in several days of down-time. In addition to the costs associated with the down-time, there is substantial cost associated with replacing the quartz and recalibrating the furnace.
An alternative to replacing the quartz inner lining is to remove the accumulated polysilicon coating therefrom. Removal of the polysilicon can be achieved with a 1:1 solution of hydrofluoric and nitric acid. The nitric acid first reacts with the polysilicon to oxidize it and produce silicon oxide. The silicon oxide is then removed by the hydrofluoric acid. Cleaning of the quartz can be accomplished every 80-100 runs of the equipment which takes approximately 2-3 weeks. The furnace must be disassembled in order to remove the polysilicon-coated quartz which is then placed into a bath of the acid mixture. The chemical reactions release heat which can cause devitrification of the quartz and render it unusable. This is especially the case when a thick film of the polysilicon is to be removed. Frequent cleaning is thus necessary. The above cleaning process requires a large amount, typically 200 gallons, of the nitric-hydrofluoric acid mixture. In addition, the method requires a substantially large area for disassembling the equipment and for cleaning the quartz.
It would thus be desirable to have a method that does not require disassembling and then reassembling the furnace for the purpose of cleaning the quartz inner lining. Avoiding this would dramatically reduce the amount of very costly equipment down-time.